When asked to identify a structure that affected their ability to communicate, most firefighters may properly include their dispatch center or a tower site. While these would be appropriate responses, because these facilities provide significant support to the radio system, attention must also be given to those buildings that have a negative effect.
Although not often considered in this light, the reduction of communications capacities caused by the design and materials of some buildings can be as deadly to firefighters as an overloaded lightweight truss. While the presence of the latter may not be readily known without a vigorous pre-plan inspection, so too may hidden radio dead spots evade detection without some firsthand knowledge of the structure involved. In either case, failure can prove costly.
Because radio waves are invisible, it is sometimes difficult to comprehend how they work, let alone what can keep them from working. Exploring the issue from a fire service perspective, we can look at the flow of electrons in a radio wave much the same way we look at the flow of water. If there is no obstruction in the hose between the pump and the fire, and if there is sufficient pressure to overcome the friction loss, the attack crew will have a viable stream. Similarly, if there is no "kink in the line" between the transmitter and the fireground, and the signal is strong enough to overcome the electronic friction loss created by man-made and natural phenomena, two-way communication will be possible.
Just as water flow can be impacted by a variety of factors such as hose diameter, elevation, and the number and condition of the fittings used in the evolution, many factors can hamper effective radio transmissions. In lower frequencies, atmospheric conditions can create a condition known as "skip," whereby calls made in one jurisdiction bounce along the atmosphere, often to be heard more clearly hundreds of miles away than they are just around the corner. On the other end of the spectrum, higher frequencies can be adversely affected by heavy foliage, meaning that a marginal location in the winter may be completely unreachable come spring. Even rain and snow can cause significant disruption to microwave signals used to tie transmitter sites together or to carry data.
In addition to the natural causes, there are several structural considerations that can reduce the usefulness of firefighting communications. One of these is the density of the structure. In designing radio systems, vendors typically refer to "light," "medium" and "heavy" construction, which roughly correspond to residential, general commercial/multiple dwellings, and large industrial, mercantile and institutional structures, respectively. As firefighters, we recognize the different building materials used in these applications. But just as a wood-frame home will behave differently toward fire than will a steel and concrete high-rise, each will also exhibit a unique resistance to radio waves.
As can be expected, the lightweight home will present little barrier to the use of radios in a properly designed communications system, but not all residences fall into the lightweight category. In fact, some that may normally not be considered as substantial can create problems. Tests conducted on National Weather Radio signals in Florida found that mobile homes and steel studs significantly hamper proper reception. This is bad news for any department using high-band VHF equipment, which is essentially the same spectrum utilized by the National Weather Service.
Almost any significant use of metal building materials - including aluminum and steel siding - can have a negative effect. It stands to reason, therefore, that larger buildings that employ a large amount of steel framing will also be poor locations for radio reception. Here, too, other factors may also come into play. Just as water flow degrades over distance, so do radio waves. A building that contains significant floor space and has numerous walls and partitions may exhibit interior dead spots. Even office furniture, such as metal desks and filing cabinets, as well as store fixtures and machinery can all help to diminish the signal.
While much was made of communications problems at the World Trade Center on 9/11, Los Angeles firefighters faced many of the same challenges 13 years earlier at the First Interstate Bank; one of the first major high-rise incidents in the United States. However, the World Trade Center operation, by its sheer magnitude, served to dispel some commonly accepted principles. It has often been assumed that the use of "talk-around," or simplex, frequencies would provide adequate in-building communications when repeaters could not be reached. The experience of the FDNY, as attested to in hearings following 9/11, was that the effectiveness of talk-around channels was spotty.
Many stories have been told over the years of firefighters having to go to a window not to get air, but rather to get a radio signal. Since glass is a very poor insulator, it may be possible to carry on a clear conversation near the outside walls of a building, yet be completely unreadable farther inside. However, as is often the case, there are notable exceptions. Some glass covered with reflective film is much more resistant to the passage of energy - including radio waves - and can seriously degrade communications.
Another factor that comes into play is distance below grade. While going deeper into the interior of a heavy building can cut down performance, so too can going deeper underground. Such situations are often encountered in parking garages, subway stations, tunnels and structures with sub-basements.
One of the latest entries into the list of potential problems concerns energy-absorbent panels that are designed to prevent the use of cellular phones in places of public assembly. A product of expressed desires to have "cell-free" zones in which to dine or watch a movie, this technology effectively blocks wireless carriers. Unfortunately, since many wireless telephones operate in frequencies immediately adjacent to those utilized by public safety, fireground communications can also be compromised.
One building-related problem can also come from height. Although in these cases, the impediment is often indirect. A large concentration of tall structures can cause a shadow effect that can block out or diminish signals to a defined area. In this scenario, a man-made mountain is created that gets in the way of radio waves that once flowed freely. This issue can arise where significant construction occurs after the radio system is already designed and installed. For this reason, repeated radio networks in urban areas may require more transmit and/or receive sites than a corresponding rural or suburban area of similar size and terrain.
Likewise, a single tall building can create concern when it blocks the path of a microwave link between public-safety sites. Microwave is often used as a reliable alternative to telephone lines where interconnection is required between transmitters, as in simulcast systems. Because microwave antennas are dishes, they produce significant wind resistance, and are typically mounted as low as possible on a tower to present a clear path while reducing the strain on the structure. If a building suddenly "pops up" in this line-of-sight-dependent path, the link will have to be relocated and reengineered.
Buildings can also cause problems when they house radio equipment or devices that can generate strong electro-mechanical fields. With cellular devices now commonplace, wireless transmitters are sometimes located in penthouses or on rooftops. Some cellular frequencies are immediately adjacent to â€” and in some cases intermixed between â€” those in the 800-MHz public-safety band. Currently, a national "rebanding" is underway to restructure these channels. However, it should be noted that one of the earliest complaints concerning wireless interference came as a result of lost transmissions at a fire station in the shadow of a cellular site. The same situation is possible when operating in structures that contain similar equipment.
Anytime that a fire department engages in an interior attack, there are certain inherent risks. However, the incident commander is responsible for calculating these risks to assure that they do not outweigh the benefits. The inability of interior crews to hear an evacuation order or to transmit a Mayday because of building construction and design should always be included in these calculations.
BARRY FUREY, a Firehouse contributing editor, is director of the Raleigh-Wake Emergency Communications Center in North Carolina. During his 35-year public safety career, he has managed 911 centers and served as a volunteer fire officer in three other states. In 2002, Furey chaired the Association of Public-safety Communications Officials (APCO) International conference in Nashville, TN, and in 2005 he received an APCO life membership for his continued work in emergency communications.